1. the redox ladder 1 0.5 0 -0.5 E h (V) O2O2 H2OH2O NO 3 - NO 2 - NH 4 + Mn + 4 Mn + 2 FeOOHFe +2 SO 4 - 2 HS - CO 2 CH 4 H+H+ H2H2 HCOO - CH 2 O clockwise: spontaneous, can produce free energy (catabolic) ccw: requires free energy (anabolic) half-reaction coupling: Nir Krakauer 2/’04

2. Oxygen units In air (sea level): 0.21 atm = 160 Torr = present atmospheric level (PAL) In water at equilibrium with PAL: 9 ml/l at 0 °C, 5 ml/l at 25 °C

3. Geochemical evidence for atmospheric O 2 >2.3 Gy BP: detrital UO 2, FeCO 3, FeS 2 ; photolytic? Mass-independent fractionation of S → O 2 at <~0.01 PAL (Berkner and Marshall [1965]: photolysis of H 2 O generates <<10 -3 PAL) 2.3> Gy: red beds, MnO 2 fields → O 2 at >0.01 PAL

4. Oxygen in the Proterozoic Canfield and Teske (1996) argue based on sedimentary S isotopes for around 0.1 PAL in the Late Proterozoic, so that there would be just enough O 2 to oxidize sulfide on shelf bottoms Anbar and Knoll (2002):

5. Eukaryotes evolved in an oxic world Eukaryote anaerobic respiration uses organic electron acceptors like pyruvate, so that it is inefficient Sterols, eukaryotic cell membrane constituents, are always made with O 2 The first eukaryotes likely didn’t have plastids and couldn’t produce O 2 Aerobic respiration can occur quite well at ~0.01 PAL O 2, the Pasteur point

6. so why aren’t there big eukaryotes much before the Cambrian? Berkner and Marshall (1965): not enough oxygen for land and sea surface UV shielding Towe (1969): making collagen demands a lot of oxygen Rhodes and Morse (1971): products of anaerobic metabolism inhibit calcification Runnegar (1981): oxygen levels not high enough to diffuse into complex organisms Anbar and Knoll (2002): metal and N limitation